Case hardening super high speed steel

ABSTRACT

A LOW CARBON, CASE HARDENED HIGH SPEED STEEL COMPOSITION AND ARTICLE HAVING A SURFACE HARDNESS IN EXCESS OF 70 ROCKWELL C IS PROVIDED CONSISTINF ESSENTIALLY OF: CARBON, 10 UP TO ABOUT .30% CHROMIUM,ABOUT 4% MOLYBDENUM, ABOUT 3-10% TUNGSTEN ABOUT,11 2-7% VANADIUM ABOUT, 1-3% COBALT ABOUT, 4-12%   THE BALANCE IS SUBSTANTIALLY IRON AND CARBURIZED AT 1725* F., HARDENED AND TEMPERED.

1974 R. F- HARVEY ET OASE HARDENING SUPER HIGH SPEED STEEL 1 Filed April21. 1972 F'n c1.

Aus-rezNn-lc GRAIN SIZE 25.0

CARBURIZED AND HARDENED CAsE N IT'AI. E-rcH IQOOX United States Patent3,827,923 CASE HARDENING SUPER HIGH SPEED STEEL Richard F. Harvey,Orchard Lake, and George E. Moore, Farmingtou, Mich., assignors to SunSteel Treating, Inc.

Continuation-impart of abandoned application Ser. No.

844,580, July 24, 1969. This application Apr. 21, 1972,

Ser. No. 246,149

Int. Cl. C22c 35/24 US. Cl. 148-315 Claims ABSTRACT OF THE DISCLOSURE Alow carbon, case hardened high speed steel composition and articlehaving a surface hardness in excess of 70 Rockwell C is providedconsisting essentially of:

Carbon, .10 up to about .30%

Chromium, about 4% Molybdenum, about 3-10% Tungsten about, 1' /2-7%Vanadium about, 1-3% Cobalt about, 4-12% The balance is substantiallyiron and carburized at 1725 F., hardened and tempered.

This application is a continuation-in-part of our copending applicationSer. No. 844,580, filed July 24, 1969, now abandoned.

This invention relates to high speed steel articles and compositions andparticularly to a new and improved class of high speed steels which havehigher hardness, finer grain size and improved physical properties thanconventionally available steels.

The alloys of the present invention are designed to be an improvementover the M40 or Superhard high speed steels. As is well known the M40 orsuperhard high speed steels are basically high carbon, high cobaltmodifications which were developed as an improvement over existinggrades for improved hardness, red hardnes and cutting ability. Insofaras it is known this important class of high speed steel as exemplifiedby A.I.S.I. M41, M42, M43, M44, M46, and M47 was first developed anddisclosed by Richard F. Harvey and coworkers.

In this connection the first printed announcement of this class ofsteels appeared in Metal Progress, May 1960, p. 113. Research efforts onthe high carbon high cobalt grades culminated in the issuance of Us.Pat. 3,113,862, on Dec. 10, 1963 to Richard F. Harvey and Charles W.Schuck. An excellent article on the superhard high speed steels appearedin the Tool and Manufacturing Engineer on July 1966, pp. 60 and 61. Thisarticle, entitled Superhard Tool Steels was written by Dr. S. G.Fletcher and C. R. Wendell of the Latrobe Steel Company.

In view of the foregoing an object of our present invention is toprovide a new family or class of super high speed steels and articlescharacterized by higher hardness, finer grain size, and better physicalproperties than the conventionally available high speed steels.

Another object of this invention is to provide a relatively low carbonanalysis which when case hardened results in the development of residualsurface compressive stress for greater strength and load carryingcapacity.

Another object of this invention is to provide a new class of lowcarbon, super high speed steels which when case hardened results inhigher red hardness than as compared to conventionally availableanalyses.

Other objects and many of the advantages of our invention will bereadily appreciated and will be better understood by reference to thefollowing description of the ice new class of steels together with theirimportant, unique advantages.

For decades it has been an important aim of metallurgists and tool steelproducers to make high speed steels with good combinations of hardness,red hardness, and toughness. The compositions of the present inventionrepresent a distinct step forward in the progress of high speed steelsto narrow the gap between high speed steels and cemented carbides.

We have discovered that the new steels having compositions except forcarbon which correspond to the compositions of the superhard or M40series of high speed steel, posses greater hardness, red hardness andductility when case hardnened according to the principles of theinvention.

When proceeding according to our invention, a super high speed steelcomposition is preselected as having fairly good combinations ofhardness, red hardness and ductility. These steels are then modified bya drastic reduction in the carbon to up to about .25% carbon. In thiscondition the steels are hardenable only up to about 42 Rockwell C.However on carburizing to a case carbon of about 1.20% and a core carbonof about .25 followed by heat treating, very exceptional properties areobtained. We have obtained peak hardness of Rockwell C /2 at a temperingtemperature of 1050" F. This is about 100 F. higher than the peaksecondary hardness obtained on tempering the conventional M40 series ofhigh speed steels.

Furthermore low carbon high speed steel made by the teachings of thisinvention exhibit very fine austenitic grain size in the hardness anduntempered condition. Other factors which contribute significantly togreater toughness is the low carbon, tough core and the high degree ofcompressive stress introduced in the surface of the heat treated highspeed steel.

The chemical analysis of our steel is such that the carbon content hasbeen reduced to less than one quarter of the carbon present in theconventional M40 series high speed steels. Also we find .that in orderto obtain the important exclusive advantages of this invention a balancemust be maintained between the austenite forming elements and theferrite forming elements. Furthermore the balance of the austeniteforming elements and the ferrite forming elements in the case and in thecore must be maintained within critical controlled limits to obtaindesired results as will be explained more fully hereinafter.

Listed below in Table I are six super high speed steels of the so calledM40 series.

TABLE I M40 series of super high speed steels C Cr V W Mo Co Thecorresponding low carbon compositions, tentatively designated as M60series high speed steels of the present invention are tabulated herewithin Table H.

TABLE 11 M60 series of low carbon high speed steels C Cr Va W Mo CoPercent Carbon .26

Manganese .24 Silicon .52

Chromium 3.61 Vanadium 1.69

Tungsten 4.48 Molybdenum 5.83 Cobalt 5.92

The experimental heat was forged to a 1% inch square billet which wasannealed.

A test ring 1%" OD by 1" ID was gas carburized in a pit type Leeds &Northrup Homo Carburizing furnace. The carburizing temperature was 1725F. and the time was 8 hours using 3 cu. ft. of natural gas per hour inan RX Carrier Gas. The case depth was .060" and the surface carbon was1.20%. After carburizing the test piece was cooled in air to roomtemperature followed by annealing at 1600' F.

The test piece was austenitized at 2165 F. to an intercept grain size of25.0. The photomicrograph FIG. 1 illustrates the fine austenitic grainsize of 25.0. This is quite fine for austenitized high speed steel andis considerably finer than the corresponding M40 Series of high speedsteel which exhibit austenitic grain sizes of about 13 to 16 in thehardened and untempered condition using equivalent austenitizingconditions.

On tempering the hardness of the steel made in accordance with thisinvention is as follows.

Tempered: Rockwell C hardness case 800 F.2 hrs. 60 850 F.2 hrs. 63 900F.2 hrs. 64 950 F.2. hrs. 67 /2 1000 F.2 hrs. 69 /2 1050 F.2 hrs. 70 /21100 F.2 hrs. 69

It should be noted that peak secondary hardness was obtained with atempering temperature of 1050 F. whereas steels of the M40 seriesexhibit maximum secondary hardness on tempering at about 950 F.

In FIG. 2 curve 1 represents the hardness for an M42 steel after varioustempering treatments. It Will be noted that the peak secondary hardness2 is obtained with a relatively low tempering temperature of 950 F.

Curve 3 represents the hardness response to tempering of the improvedsteel of the present invention. The peak secondary hardness 4 isobtained with a tempering temperature of 1050" F.

This indicates greater red hardness and resistance to tempering of thesteel of the present invention. An increase of 100 F. in the temperaturefor peak secondary hardness represents a significant increase inresistance to tempering which should be of interest in all elevatedtemperature applications where high speed steel is used.

We have found that it is essential that a balance be maintained betweenthe austenite forming elements and the ferrite forming elements. Thisbalance must be maintained between the case and the core for bestresults. The various elements vary in their alloying effect inaccordance with the factors tabulated in Table H.

4 TABLE H Relative Effect of Alloying Elements Austenite forming:

Carbon 35 Nitrogen 35 Cobalt l Ferrite forming:

Chromium 1 Silicon 5 Molybdenum 2 Tungsten 1 Manganese 1 Vanadium 3 Inapplying these factors, the following tabulation will illustrate theratio of the austenite forming elements to the ferrite forming elementswhich has been found to result in optimum hardness, red hardness, andfine grain size which combination is an outstanding characteristic ofthe present invention.

Table III illustrate the application of these factors for the core ofthe example of the steel of the present invention and Table IVrepresents the application of these factors for the case of the steel ofthe present invention.

TABLE III Austenite forming elements Ferrite forming elements Carbon.26X35=9.10 Chromium. Cobalt 5. 92Xl=5. 92 Silicon Manganese. .24 1=.24Total austenite Molybdenum.-..- 5. 83X2=1L 66 Alloying effect... 15. 02Tugsten 4. 48X1=4. 48 Vanadium 1. 60 3=5. 07

Total ferrite alloying effect 27. 71

Total austenite alloying effect 15.02 Norm-Ratio Total ferrite alloyingeffect 27. 71

TABLE IV Austenite forming elements Ferrite forming elements Carbon 1.20X35=42.0 Chromium 3. 61Xl=3. 61 Cobalt 5. 92X1=5. 92 Silicon.52X5=2.60 Manganese. .24X1=.24 Total austenite Tungstem. 4. 48X1=4. 48alloying effect-.- 47. 92 Molybdenum. 5.83 2=11. 66 Vanadium 1 69X3=5 07Total ferrite alloying effect 27. 71

Total austenite alloying effect 47. 92 NOEL-Ratio ----=--=1.73

Total ferrite alloying effect 27. 71

We find that the ratio of the total austenite alloying effect to thetotal ferrite alloying effect for the core in the example cited is .57and preferably this ratio should be within the limits of about 0.4 to0.8.

We find also that the ratio of the total austenite alloying effect tothe total ferrite alloying effect for the case in the example cited is1.73 and preferably this ratio should be within the limits of about 1.55to 1.95.

It will be observed also that in the example cited the ratio of thetotal austenite alloying effect in the case of the total austenitealloying effect in the core is 47.92/ 15.02=3.l9. This is a criticalratio which should be rigidly held between the limits of about 2.90 and3.50 for optimum results.

FIG. 3 illustrates the extent of the total austenite alloying effect inthe case and core for optimum properties.

The area abcd represents the useful area of compositions including thecase and core according to the teachings of the invention.

From the foregoing it will be noted that the improved class of highspeed steels of the present invention are characterized by high hardnessand resistance to softening along with a fine grain size and highductility.

While we do not wish to be bound by the limitations of a theory webelieve that the superior and unique results of this invention are duein large measure to the effect of residual compressive stressesdeveloped during case hardening. These compressive stresses are believedto be beneficial in creating higher hardness, higher red hardness and afiner grain size.

Go the basis of the foregoing our improved composition for a casehardening super high speed steel lies within the following range.

The carbon in the case after case hardening should be in the range 1.10%to 1.30%, preferably about 1.20%.

While we have set out certain preferred practices and embodiments of ourinvention in the foregoing specification it will be understood that thisinvention may be otherwise embodied within the scope of the followingclaims.

We claim:

1. A high speed steel composition characterized by having a surfacehardness in excess of 70 Rockwell C and a low carbon high strength coreconsisting essentially of about 0.15 to 0.25% carbon; 0.45% maximummanganese; 0.45% maximum silicon; 3.5 to 4.5% chromium; 1.0 to 3.0%vanadium; 1.15 to 6.0% tungsten; 3.5 to 9.5% molybdenum; 5.0 to 8.0%cobalt, with the remainder essentially iron; said composition havingbeen carburized to a surface carbon content of about 1.10 to 1.30%,hardened and tempered.

2. A high speed steel composition characterized by having a surfacehardness in excess of 70 Rockwell C and a low carbon high strength coreconsisting essentially of about 0.15 to 0.25 carbon; 0.45 maximummanganese; 0.45% maximum silicon; 3.5 to 4.5 chromium; 1.0 to 3.0%vanadium; 1.5 to 6.0% tungsten; 3.5 to 9.5 molybdenum; 5.0 to 8.0%cobalt, with the remainder essentially iron; said composition havingbeen carburized at 1725 F. to a surface carbon content of about 1.20%hardened at 2165 F. and tempered at 1050 F.

3. A high speed steel composition characterized by having a surfacehardness of over 70 Rockwell C and a low carbon high strength coreconsisting essentially of about 0.10 to 0.30% carbon; 0.60% maximummanganese; 0.60% maximum silicon; 3.0 to 5.0% chromium; 1.0 to 3.5%vanadium; 1.5 to 7.0% tungsten; 3.0 to 10.0% molybdneum; and 4.0 to12.0% cobalt, with the remainder substantially iron; said compositionhaving been carburized to form a surface hardened case having a ratio oftotal austenite alloying effect in the case to the total alloying elfectin the core of between about 2.90 to 3.50.

4. A high speed steel composition as claimed in claim 3, having beencarburized at 1725 F. to a surface carbon content of about 1.2%,hardened at 2165 F. and tempered at 1050" F.

5. A case hardened high speed steel composition consisting of about1.10% to 1.30% carbon in the case about 0.10 to 0.30% carbon in thecore, characterized by having a surface hardness of above about 70Rockwell C the balance of the composition consisting essentially of 3.0to 5.0% chromium; 1.0 to 3.5% vanadium; 1.5 to 7.0% tungsten; 3.0 to10.0% molybdenum; and 4.0 to 12.0% cobalt; the remainder beingsubstantially iron with residual elements in normal amounts; saidcomposition having an intercept grain size in the case of about 25having been hardened and tempered.

6. A case hardened high speed steel composition characterized by havinga surface hardness in excess of 70 Rockwell C and a low carbon highstrength core consisting essentially of about 0.10 to 0.30% carbon; 3.50to 4.50% chromium; 1.0 to 3.0% vanadium; 1.5 to 6.0% tungsten; 3.5 to9.5% molybdenum and 5.0 to 8.0% cobalt; the remainder beingsubstantially iron with residual elements in normal amounts; saidcomposition being characterized by a ratio of total austenite alloyingeffect to total ferrite alloying effect of between about 0.4 to 0.8; andafter carburizing being characterized by a ratio of total austenitealloying effect in the case to the total ferrite alloying effect in thecase of between about 1.55 to 1.95; said composition having beenhardened and tempered.

7. A high speed steel composition as claimed in claim 6 which has beenhardened at 2165 F. and tempered at 1050 F.

8. A case hardened article of high speed steel characterized by having asurface hardness in excess of 70 Rockwell C and a low carbon highstrength core consisting essentially of about 0.15 to 0.25% carbon;0.45% maximum manganese; 0.45% maximum silicon; 3.5 to 4.5% chromium;1.0 to 3.0% vanadium; 1.5 to 6.0% tungsten; 3.5 to 9.5% molybdenum; 5.0to 8.0% cobalt, with the remainder essentially iron; said compositionhaving been carburized to a surface carbon content of about 1.10 to1.30% hardened and tempered.

9. A low carbon case hardened article characterized by having a surfacehardness of over 70 Rockwell C and a low carbon high strength coreconsisting essentially of about 0.10 to 0.30% carbon; 0.60% maximummanganese; 0.60% maximum silicon; 3.0 to 5.0% chromium; 1.0 to 3.5%vanadium; 1.5 to 7.0% tungsten; 3.0 to 10.0% molybdenum; and 4.0 to12.0% cobalt, with the remainder substantially iron; said compositionhaving a ratio of total austenite alloying effect in the case to thetotal alloying effect in the core of between about 2.90 to 3.50 andhaving been carburized at 1725 F. to a surface carbon of about 1.20%,hardened at 2165 F. and tempered at 1050" F.

10. A case hardened article consisting of about 1.20% carbon in thecase, and about 0.10 and 0.30% carbon in the core, characterized byhaving a surface hardness of above about 70 Rockwell C on tempering at1050 F., the balance of the composition consisting essentially of 3.0 to5.0% chromium; 1.0 to 3.5% vanadium; 1.5 to 7.0% tungsten; 3.0 to 10.0%molybdenum; and 4.0 to 12.0% cobalt; the remainder being substantiallyiron with residual elements in normal amounts; said composition havingan intercept grain size in the case of about 25 on austenitizing at 2165F. and tempered at 1050 F.

References Cited UNITED STATES PATENTS 1,991,805 2/1935 Kluger -126 H2,033,927 3/ 1936 De Fries 148-315 2,081,394 5/ 1937 De GOlyer 75-126 H2,147,122 2/ 1939 Emmons 75-126 H 2,289,449 7/ 1942 Nelson 75-126 H2,676,098 4/1954 Payson 148-39 3,259,489 7/ 1966 Hamaker 148-31 HYLANDBIZOT, Primary Examiner

